This invention relates to laminated safety glass and to a polymeric laminate which includes a layer of plasticized polyvinyl butyral (PVB) for use in such structures.
The present most widely used vehicle windshield is a three ply laminate comprising an energy absorbing plastic sheet sandwiched between two glass sheets. Such windshields are capable of absorbing a blow from the head of an occupant without shattering and in doing so they dissipate a relatively significant amount of impact energy as the windshield undergoes an essentially plastic deformation.
A disadvantage of such a three ply structure is that the occupant can be subjected to facial lacerations from sharp edges of broken glass of the inwardly directed glass sheet of the windshield. To reduce this the inside glass surface has been coated or covered with a protective plastic layer intended to prevent an occupant's skin from contacting the glass and further increase the penetration resistance of the conventional three ply laminate. Such protective layer, which usually comprises two or more plies, is known as an anti-lacerative shield (ALS). Alternatively, to address this occupant laceration problem and improve vehicle fuel efficiency by reducing weight, it has been proposed to reduce the conventional three ply structure to a single glass layer having a layer of an energy absorbing material and a protective covering on its inboard side. Such a composite structure is known as a bilayer windshield and is especially suitable for ease of fabrication into relatively complex shapes.
In conventional three ply (glass/plasticized PVB/glass) laminates, the inboard glass layer provides moisture barrier protection for the PVB. In bilayer and ALS applications using plasticized PVB, the absence of this protective glass layer has been recognized as a problem. As stated in U.S. 4,584,229 (col. 1, lines 17-33), the optical quality and PVB adhesion to glass unacceptably deteriorates over time from absorption of atmospheric moisture by the PVB layer. Thus, at the equilibrium moisture content in an atmosphere of about 50% relative humidity (RH), PVB sheet conventionally used in three ply laminates absorbs about 1 % moisture which results in an unacceptable low level of adhesion to glass. Published U.K. specification No. 1,394,271 proposes a sealing or moisture barrier layer between the plasticized PVB and an inboard wear-resistant reinforcing layer (i.e. a layer sequence of glass/plasticized PVB/sealing layer/reinforcing layer) to prevent diffusion of water vapor through the reinforcing layer into the PVB layer. Such moisture barrier layers are described as polyvinylidene chloride (PVDC), polyvinyl chloride, polyolefins and the like. Unfortunately, as recognized by the present inventors and later further discussed herein, these barrier layers only reduce the rate but do not eliminate atmospheric water vapor transfer into the PVB. For example, 2 mils (0.05 mm) thick PVDC, which is one of the most impervious of commercially available moisture barrier films, adhered to 45 mils (1.14 mm) thick conventional commercially available plasticized PVB sheet in 80% relative humidity, which is representative of that encountered during summer months in New Orleans, LA, only provides protection for 6-7 weeks before the moisture content of the PVB sheet reaches about 1%.
As typically represented by the '229 patent, for bilayer and ALS structures the prior art proposes spraying special adhesive polyurethanes onto the glass layer over which is then deposited a different, wear-resistant inboard polyurethane layer. However, sprayed liquid adhesive coatings are not usually encountered in commercial glass laminating systems which have long been based on polyvinyl butyral with which laminators are quite familiar in handling and use. Moreover, plasticized PVB as an energy absorbing layer has performance advantages over polyurethane. For example, the impact tear resistance of PVB is greater than polyurethane over most of the operating temperature range encountered in vehicle window use. At thicknesses providing the expected degree of safety in use, the component cost of an all-polyurethane system is about twice that of one containing PVB as a layer. Optical clarity can and has been designed into plasticized PVB sheet supplied by sheet manufacturers to glass laminators which may be more difficult to achieve when being developed by depositing liquid polyurethane susceptible to uneven gravity flow on a contoured window panel. A laminate having plasticized polyvinyl butyral as a layer for laminating to a glass sheet in bilayer or ALS applications would be desirable.